This paper presents strong experimental evidence for a major perturbation in ozone
concentrations over large parts of the North Atlantic Ocean from the surface to 8 km associated with continental pollutants. The evidence was gathered in the course of 7
flights by the UK Meteorological Office C-130 aircraft based on the Azores, and 4 ferry flights between the UK to the Azores in spring and summer 1997 as a component of
the NERC-funded ACSOE project. The total latitude range covered was approximately
55°N–25°N, and the longitude range was approximately 0° to 40°W. Many profiles
were made between the sea surface and altitudes up to 9km to survey the composition of the marine atmosphere.
The C-130 aircraft was comprehensively equipped to measure many chemical and
physical parameters along with standard meteorological instrumentation. Thus it was
able to measure ozone and speciated NOy, along with tracers including water vapour,
carbon monoxide and condensation nuclei, in near real time. The overall ‘picture’ of the
troposphere over large parts of the North Atlantic is of layers of pollution from the continents of different ages interspersed with layers of air uplifted from the marine boundary layer. The lowest ozone concentrations were recorded in the marine boundary layer where there is evidence for extensive ozone destruction in summer.
Flights were made to penetrate the outflow of hurricane Erica, to determine the
southerly extent of polluted air in summer, to examine the impact of frontal systems
on the composition of remote marine air, and to trace long-range pollution from the
west coast of the USA interspersed with air with a stratospheric origin. In one of the
spring flights it is possible that a plume of polluted air with high ozone and NOy, and
with an origin in southeast Asia, was intercepted off the coast of Portugal. The concentrations of NOx in this plume were sufficient for ozone formation to be continuing along its track from west to east.
The instrument to measure NOy almost certainly was only measuring the sum of organic nitrates (mostly in the form of PAN) plus NOx. The high correlation between NOy and ozone under these conditions strongly suggests a non-stratospheric source for most of the ozone encountered over large parts of the atmosphere upwind of Europe.
There was a marked seasonal variation in the NOy with about twice as much present
in the spring flights than in the summer flights. The overall ozone levels in both spring and summer were somewhat similar although the highest ozone concentration encountered ~100 ppbv) was observed in summer in some polluted layers in mid Atlantic with an origin in the boundary layer over the southeastern USA.
The bulk of the pollutants, ozone, CO, and NOy, were in the free troposphere at altitudes between 3 and 8 km. The only instances of pollution at lower levels were in the form of ship plumes, which were encountered several times. The data therefore strongly support the need for more in-situ aircraft experiments to quantify and understand the phenomenon of long-range transport of pollution from continent to continent.
Observations at ground-based stations are inadequate for this purpose and satellite
data is incomplete both in terms of its altitude detail and in the extent of chemical speciation, particularly for ascertaining whether chemical production and destruction processes for ozone are occurring.